The present invention relates to an infrared laser beam viewing apparatus used in connection with inspecting and improving the quality of a laser beam emitted from a laser.
Infrared lasers, such as Neodymium yttrium-aluminum-garnet (Nd-YAG) lasers are used for scribing identification marks on semiconductor wafers. These marks can be located anywhere on the planar surfaces of a wafer, and can be alphanumeric, bar code, or any other desired marking pattern. In manufacturing satisfactory marks, the power intensity, location, and focus of the laser beam must be carefully controlled. To scribe wafers, the laser beam will leave the generator, and pass through a path of several mirrors and lenses before reaching the marking surface. Since the current art for marking semiconductor wafers requires marking tolerances within microns, laser beam placement, focus, and power intensity are critical.
One of the problems inherent with scribing wafers is properly controlling the quality of the laser beam as it passes through the various optics. Ideally, the laser beam should be aligned so that the center of the laser beam strikes or passes through the center of the various optics along its path, and that the optics used are defect free. Keeping the center of the laser beam in the center of the defect-free optics reduces distortion of the laser beam and minimizes power loss, whereas optics with defects, or optics that are not aligned properly, produce a distorted beam which increases power usage and inhibits or prohibits achieving required tolerances. However, the beam produced by a Nd-YAG laser falls in the infrared light spectrum, and cannot be seen by the naked eye. Therefore, to align the laser beam and check the beam quality through the optics, either a phosphor-covered disk or an infrared viewing scope may be used to track the location of the laser beam.
Unfortunately, neither of these methods for viewing the laser beam produces a sharp image of the periphery of the laser beam. Neither tool provides the ability to distinguish power variations across the beam. As illustrated in FIG. 1, the power intensity across the beam produces a gaussian curve with the highest power found at the center of the beam, and power decreasing normally on both sides. Unfortunately, neither the viewing scope nor the phosphor block allows this gaussian curve to be seen. Therefore, if there is an occlusion in an optic, or the alignment is not centered on the optic, this curve will be altered, thus changing the predicted functionality of the beam. Since the marking process for semiconductor wafers requires tolerances in the micron range, it is imperative that the laser beam be known and properly characterized. As a result of a non-normal shaped curve during the marking process on a wafer, the width and depth of the mark are impossible to control, thus the mark will not meet the required specifications.
The phosphor-covered disk, such as the Near IR Display Plate Model 24, manufactured by Optical Engineering, Inc., when held in the direct path of the laser beam, produces a visible green image where the beam strikes the phosphor. There are some significant disadvantages associated with this technique however. First, as illustrated in FIG. 2, the sensitivity of the block is such that it is only able to display beam power at or above approximately 0.3 watts, which prohibits viewing the outer periphery of the beam. Another distinct disadvantage of this method is that the phosphor block becomes saturated almost immediately upon reaching the threshold of visibility. Therefore, the central part of the beam that is viewable all appears the same, and the variable power area is so small that no details can be determined about power variability in beam. Finally this method requires that the beam strike the phosphor directly, and therefore only provides information about the beam where it strikes the phosphor. In each of FIGS. 2, 3, and 5, area 5 represents the saturated area of the beam intensity viewing method, areas 3 and 4 are the variable intensity regions where the intensity increases as it moves toward area 5, and areas 1 and 2 are the areas below the threshold of visibility.
An alternate method of viewing the beam uses an infrared viewing scope, a camera-like device that produces an image on a monitor displaying where the beam contacts an optical surface such as a mirror or lens. One such infrared viewing scope is the Infrared Electroviewer Model 7215, manufactured by Electrophysics Corp. Although the infrared viewing scope provides the opportunity to observe the beam as it interacts with the surface it is striking, it also has the disadvantage of being extremely sensitive to power output. As illustrated in FIG. 3, the infrared viewing scope only displays the part of the beam that is absorbed within the glass of the optic it strikes, which is usually between 5% and 10%. This small amount does not satisfactorily provide information about the quality of the beam. The infrared viewer has a very low power threshold of approximately 0.003 watts, but similar to the phosphor block, it saturates at a power level very close to the threshold. The image on the infrared viewer appears the same for any power higher than the saturation point, again providing little or no information about the quality of the beam in the outer periphery.
Filters have been used to assist in inspection of peripheral edges of laser beams by increasing the range of visible-to-saturation thresholds. These lenses have historically been manufactured out of glass that is manufactured to very exacting standards for shape and surface smoothness, among other parameters. These filters work reasonably well in a controlled environment where safety can easily be controlled, but are dangerous for use in manufacturing facilities where there are many people, and protective equipment is limited. When a laser beam strikes a filter medium, a portion of the beam is reflected. With a glass filter medium with a smooth surface, the reflected beam is not dispersed, but rather is often concentrated, and very dangerous for people working in proximity to the laser. Also, filter mediums also absorb some of the energy from the beam, and occasionally the filter will shatter, sometimes very violently.
Accordingly, a need has developed that allows a laser beam operator to view and monitor the peripheral edge of the laser beam for imperfections caused by such things as power problems or occlusions in one or more optical components, as the beam interacts with the optical components using a filter medium that will disperse reflected light, and will not shatter.